Is it cruel to give a cat catnip? Cats go after the herb by themselves, but too much can make them aggressive.
Would it be considered halal or haram to give a cat catnip?
Just for fun? Does it annoy them?
I assume you are not sure because it has no benefit to the animal and will annoy them. Annoying animals and other creatures without any reason is definitely not pleased by God.
I found this Hadith a really great example, to what degree the rights of animals is important in Islam. It is from Nahj-Al-Balagha, Sermone 224.